1. Field of the Invention
The present invention relates to a magneto-optical recording medium, and specifically to a magneto-optical recording medium utilizing displacement of magnetic domain walls at the time of reproduction.
2. Related Background Art
Various magnetic recording media have been practically used as rewritable recording media.
In recent years, requirement for an increase in recording density of the magnetic recording medium to produce a recording medium having a larger capacity has been growing along with the progress of digitization of moving images. In particular, a magneto-optical recording medium having a magnetic thin film into which magnetic domains are written using thermal energy from a semiconductor laser to record information and from which the information is read out using a magneto-optical effect has been expected as a rewritable recording medium which is recordable at a high density and has a high capacity.
In general, the linear recording density of an optical recording medium largely depends on the wavelength of a laser of a reproducing optical system and the numerical aperture NA of the object lens thereof. That is, when a wavelength λ of the laser of the reproducing optical system and the numerical aperture NA of the object lens thereof are determined, the diameter of a beam waist is determined. Therefore, a limit of a spatial frequency of recorded pits for reproducing a signal becomes about 2 NA/λ. Thus, when a density in a conventional optical disk is to be increased, it is necessary to shorten the wavelength of the laser of the reproducing optical system or to increase the numerical aperture NA of the object lens thereof.
In view of the wavelength of the laser, a blue semiconductor laser having a short wavelength of about 405 nm and about 435 nm has been developed and practically used in contrast to a conventional red semiconductor laser having a wavelength of about 635 nm to 685 nm. However, when the wavelength of the laser is further shortened, it is within an ultraviolet range, raising a problem in that a material transmitting ultraviolet light is limited. Therefore, it is expected to become very hard to further shorten the wavelength for recording and reproducing. On the other hand, when the numerical aperture of the object lens increases, a focal depth becomes shallow in proportion to the square of the numerical aperture, so that a permissible variation in interval between a disk serving as a recording medium and a pickup, that is, a permissible runout of the disk acceleratedly reduces. As a result, a focal servo or the like is required for the pickup, so that a problem occurs in that tight requirement is made for mechanical precision.
Therefore, it is important to realize a high density without a change in numerical aperture of the object lens even when the recording density is improved by the shortening of the wavelength of the laser. Thus, various high-density techniques for improving the recording density by devising a structure of a recording medium and a reproducing method have been developed.
For example, according to Japanese Patent Application Laid-Open No. H6-290496, a magneto-optical recording medium and a reproducing method therefor have been proposed, in which a magnetic domain wall present in a boundary portion between recording marks is displaced to a high temperature side utilizing a temperature gradient, and the displacement of the magnetic domain wall is detected, whereby a signal of a recording density which exceeds resolution of an optical system can be reproduced without a reduction in amplitude of a reproducing signal.
In order to smoothly displace the magnetic domain wall, a proposal of disconnecting magnetic coupling between adjacent recording tracks has been made in, for example, Japanese Patent Application Laid-Open No. 2002-150631. According to Japanese Patent Application Laid-Open No. 2002-150631, laser annealing for irradiating a portion between the recording tracks with high-power laser light is performed to change a magnetic layer between the tracks to an in-plane magnetized film, whereby preferable reproduction in which a magnetic domain is extended due to the displacement of the magnetic domain wall is realized. According to Japanese Patent Application Laid-Open No. 2002-203343, a proposal has been made of annealing a magnetic film from an opposite side to a substrate while tracking control is performed by a sample servo system.
According to Japanese Patent Application Laid-Open No. 2003-317336, a proposal has been made of using land/groove recording to narrow a track pitch and annealing a side wall portion between a groove and a land as a recording track.
The inventors of the present invention found that a variation in reproducing signal characteristic is caused in a conventional magnetic domain wall displacement magneto-optical recording medium in which laser annealing is performed on a portion between recording tracks. As a result of pursuing this cause, the inventors determined that the following matter is likely to be a main factor. A magnetized state of the annealed region is supposed to fundamentally become an in-plane magnetized state. However, the annealed region becomes a region where a perpendicular magnetized film and an in-plane magnetized film coexist due to the influences (leakage magnetic field and exchange coupling) of magnetized states of adjacent recording tracks. In addition, the region which has become the perpendicular magnetized film is in a state that a region magnetized upward and a region magnetized downward coexist.
In other words, a variation in displacement of the magnetic domain wall is caused by a random magnetized state of the laser-annealed region, thereby resulting in a variation in reproducing signal characteristic occurs.